The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which include gerotor displacement mechanisms.
Although the present invention may be used advantageously with gerotor devices which are to be used as fluid pumps, the invention is especially advantageous when utilized as part of a gerotor motor, and will be described in connection therewith.
Furthermore, although the present invention may be used advantageously with gerotor motors having various types of valving, it is especially advantageous when utilized in a high pressure motor of the "valve-in-star" (VIS) type, and will be described in connection therewith. An example of a VIS motor is illustrated and described in U.S. Pat. No. 4,741,681, assigned to the assignee of the present invention and incorporated herein by reference. In a VIS motor, commutating valving action is accomplished at an interface between an orbiting and rotating gerotor star, and an adjacent, stationary valve plate, which is typically part of the motor housing.
More specifically, the present invention relates to a gerotor motor of the "wet-bolt" type, an example of which is illustrated and described in U.S. Pat. No. 5,211,551, also assigned to the assignee of the present invention and incorporated herein by reference. In a welt bolt motor, there are seals disposed between the various sections of the motor, and the fasteners (typically, bolts) which hold the sections of the motor in tight sealing engagement are disposed radially inwardly from the seals. Therefore, such a motor is referred to as a wet-bolt motor because leakage fluid flowing between adjacent sections of the motor can enter the bolt bores, then flow axially along the bore to a case drain region, etc. The use of a "wet-bolt" design in a gerotor motor is a way to reduce the size and weight, and therefore, the cost of the motor and is generally a desirable approach.
The VIS motors illustrated and described in the above-incorporated patents are high pressure, high performance motors, and it has been determined that performance characteristics such as the volumetric efficiency are improved by the use of a balancing plate, disposed adjacent the "forward" end of the gerotor, i.e., opposite the end of the gerotor where the commutating valving action occurs.
As is well known to those skilled in the art, a major cause for the loss of volumetric efficiency is difference between the "height" (i.e., axial length) of the gerotor ring and star, this difference being referred to as the "side clearance". It has been typical practice to make the star somewhat shorter than the ring, primarily to accommodate possible growth of the star caused by thermal shock (i.e., the sudden introduction of hot oil into a cold motor).
Unfortunately, the attempts by those working in the art to maintain a very small "side clearance" tolerance band, of the star relative to the ring, have always added substantially to the total manufacturing cost of gerotors of the type which are used in low speed, high torque (LSHT) motors, and especially those which are intended for high pressure and high performance.